In confocal microscopy, an illumination source is focused onto a point at the sample, and then an image of the sample point is formed at an aperture.
The focal point of the illumination source is scanned across the sample, and the light intensity at the aperture is used to form an image, with the intensity of each image point corresponding to the intensity of light passing through the aperture. By focusing the illumination source at different depths within a sample, images from different depths in the interior of a transparent sample can be formed because the small aperture blocks electrons scattered from depths other than the focal depth.
Confocal microscopy was developed using light microscopy, but it has recently been applied to electron microscopy. In conventional confocal electron microscopy, a disc detector is placed at the aperture in a plane conjugate to the specimen. The size of the disc detector as projected back to the specimen plane is about equal to the lateral resolution of the confocal image, which is typically a few Angstrom. Because the beam must scan across the sample, it is necessary to align the detector below the sample with the beam above the sample. U.S. Pat. No. 6,548,810 to Zaluzec for “Scanning Confocal Electron Microscope” describes the use of “descan” coils to bring the beam that has passed through the sample back onto the optical axis to pass through an aperture and into a detector.
Because of the very small disc detector used in confocal microscopy, the de-scanning and the re-focusing of the beam is not sufficiently precise to provide for atomic resolution. To overcome this problem, recent publications in the field of scanning confocal microscopy describe a mechanism in which the sample moves under the beam, instead of the beam moving over the sample. For example, Takeguchi et al., “Development of a stage scanning system for high resolution confocal STEM,” Journal of Electron Microscopy 57(4) 123-127 (2008) describes a confocal electron microscope in which the sample is mounted in a goniometer that uses piezoelectric actuators to move the sample. By scanning the sample in a rectangular raster pattern, the microscope can form a two dimensional image of a plane of the sample. By moving the sample up or down, information from multiple planes inside the sample can be acquired to form a three-dimensional image. Mechanical scanning, however, is much slower than scanning the electron beam.
Similarly, Cosgriff, D'Alfonso, et al., “Three-dimensional imaging in double aberration-corrected scanning confocal electron microscopy, Part I: Elastic Scattering,” Ultramicroscopy 108 (pp. 1558-1566 2008) shows a sample that can be moved in an X-Y plane as well as in the Z direction, to form a three dimensional image of the sample.
D'Alfonso, Cosgriff, et al, “Three-dimensional imaging in double aberration-corrected scanning confocal electron microscopy, Part II: inelastic Scattering,” Ultramicroscopy 108 (pp. 1567-1578 2008) uses a scanning confocal electron microscope to determine the location of isolated impurity atoms embedded within a bulk matrix. D'Alfonso identifies an individual impurity atom using the electron energy loss from inner shell ionization and detects the energy loss electron by directing a defocused electron toward a detector.